The contrast in an X-ray image is usually caused by different attenuation properties of the substances relative to the X-radiation produced by an X-ray machine. When examining a patient, for example, it is possible on the basis of the different attenuation properties of bone tissue and soft part tissue to analyze the structure of the bone in the body interior of a patient on the basis of the contrast. Organs or vessels having an attenuation property similar to the surroundings cannot be examined in a conventional way because of too low a contrast.
For this reason, a contrast agent is used when examining an organ supplied with blood, for example, a heart or a liver. The contrast agent exhibits a different attenuation property by comparison with the surrounding tissue and so a visible contrast is produced between the organ and the surroundings in the image.
The propagation of the contrast agent in the patient's body is a highly dynamic process. After a certain time, the concentration of the contrast agent in an examination region being viewed rises steeply at first, reaches a maximum and subsequently falls back again. Thus, as the examination region is scanned at the concentration maximum, a preliminary examination with a small quantity of the contrast agent (test bolus) is firstly carried out at the beginning of each examination. The temporal behavior of the contrast agent concentration is represented in the form of a contrast agent curve on the basis of which the operating parameters of the X-ray machine are determined for carrying out the scanning. For example, the time suitable for starting the scanning can be calculated from the time interval between introducing the contrast agent and the observed maximum in the concentration.
If a further examination with the aid of a contrast agent is required for a patient at a later time, it is necessary, in turn, for an additional preliminary examination to be carried out with the aid of a test bolus in order to determine the operating parameters of the X-ray machine.